This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The goal of the proposed research is to develop 1s2p resonant inelastic x-ray scattering (RIXS) as a general and quantitative tool for studying enzyme intermediates. We have previously used metal L-edge spectroscopy at SSRL to extend the concept of metal-ligand covalency by showing that the L-edge band shape can be used to quantify interactions between ligands and metal d-orbitals of different symmetry, i.e., differential orbital covalency (DOC). The DOC method allows for experimental determination of basic concepts in chemistry. However, metal L-edge spectroscopy is performed with soft x-rays in ultra-high vacuum and cannot in general be used for enzyme intermediates. 1s2p RIXS involves excitation from 1s 3d (h ~ 7.1 keV) and monitoring of the subsequent 2p 1s photon emission (h2 ~ 6.4 keV). The final state in this resonance process is 2p53dn+1, the same final state as for the L-edge experiment. 1s2p RIXS uses hard x-rays and does not require ultra-high vacuum, and can therefore be used to obtain L-edge-like spectra under in situ conditions. Our new contribution is a quantitative analysis of RIXS spectra in the energy transfer (h - h2) direction using the DOC analysis. The long life time of the 2p hole results in a sharp and feature-rich spectrum that can be accurately fitted using a parameterized multiplet model that includes configuration interaction. The electronic structure, including the DOC, is subsequently extracted from the theoretical model. The method will be used to study high-valent enzyme intermediates that activate inert C?H bonds;heme compounds I and II, the non-heme ferrryl-oxo intermediate in SyrB2 and intermediate Q in the binuclear iron enzyme methane monooxygenase. RIXS provides a new tool to investigate the requirements for C?H bond activation, but also to explain differences in reactivity between heme and non-heme enzymes.